The folate antagonist methotrexate (MTX) has been in clinical use for over 20 years and has an established role in the chemotherapy of several types of cancer. In recent years it has been shown that the clinical usefulness of MTX can be extended to additional types of cancer and the antitumor effects can be enhanced by the technique of high-dose MTX and citrovorum factor (CF) "rescue". However, high-dose MTX/CF rescue is associated with a high incidence of toxicity and some fatalities. At the present time there is no generally accepted protocol for CF rescue. The great variability in clinical protocols for CF rescue and the high incidence of toxicity reflect our lack of knowledge of the pharmacologic basis for rescue and a lack of pharmacokinetic data on CF to enable one to design rational rescue protocols. A primary objective of the proposed studies is to determine the pharmacokinetics of CF and its active metabolites in dogs. A significant part of this study will be concerned with determining the mutual effects of CF and MTX on their individual pharmacokinetic behavior. Dogs will be used for these studies since studies of MTX pharmacokinetics and renal function in general in this species are known to correlate with human studies. Another aim is to determine the in vivo concentration of CF required for rescue of G.I. mucosa and bone marrow from various concentrations of MTX. Additional objectives are directed toward further development of an alternative method of rescuing from MTX using thymidine (Td) and/or purines to bypass the biochemical lesion produced by antifolates. These studies have as their goal the determination of the pharmacokinetic data base for using this method of rescue and determining the concentration of TdR and/or purines required to protect normal tissues. In addition, these studies will utilize tumor-bearing mice in an attempt to obtain a better understanding of the biochemical, pharmacokinetic, or cell kinetic basis for the enhanced antitumor activity seen with high-dose MTX/CF rescue.